Conventionally, there has been known a needle assembly comprising a hub retaining a cannula, a protective cover advancing and retracting with respect to the hub to be situated at an advanced position where it surrounds the forward end of the cannula and a retracted position where it exposes the forward end of the cannula, an urging means elastically provided between the hub and the protective cover and urging the protective cover forwards, and a stopper means retaining the protective cover at the advanced position.
Further, as an example of such a needle assembly, there is also known a needle assembly in which the protective cover is moved to be placed in a pre-use state in which it is situated at the advanced position to surround the forward end of the cannula, a use state in which the protective cover is situated at the retracted position to expose the forward end of the cannula, and a post-use state in which it is again situated as the advanced position to surround the forward end of the cannula and in which it is fixed in position so that it may not move to the retracted position again (Patent Documents 1 and 2).
The insulin injection system of Patent Document 1 is composed of the above-mentioned hub, the protective cover provided so as to cover the outer peripheral surface of the hub, and a spring member provided inside the hub and abutting the forward end side bottom portion of the protective cover.
In the insulin injection system of Patent Document 1, an engagement protrusion is provided on the inner peripheral surface of the protective cover, and a guide groove guiding the engagement protrusion is provided in the outer peripheral surface of the hub.
When the protective cover in the pre-use state is pressed backwards, this guide groove guides the engagement protrusion backwards, and, when the cannula is placed in the use state, it moves forwards while rotating along a groove formed obliquely forwards to be placed in the post-use state.
In the post-use state, a protrusion is fit-engaged with a recess formed in the guide groove, preventing the protective cover from moving backwards again.
The injection needle structure of Patent Document 2 is composed of a cylindrical portion surrounding the outer periphery of the protective cover in front of the hub, a fixation member provided at the rear of the protective cover, and a spring member elastically provided between the fixation member and the hub.
In the injection needle structure of Patent Document 2, a protrusion is provided on each of the inner peripheral surface of the cylindrical portion, the outer peripheral surface of the protective cover, and the outer peripheral surface of the fixation member, with these protrusions being inclined as appropriate.
Thus, when the protective cover is retracted from the pre-use state, the protective cover rotates to be placed in the use state, with the inclined surfaces of the protrusions sliding on each other; thereafter, when the protective cover moves forwards, the protective cover rotates to be placed in the post-use state, with the inclined surfaces of the protrusions further sliding on each other.
In this post-use state, the protrusions are held in contact with each other, preventing the protective cover from retracting.